For a review of the applications of such lasers, reference can be made for instance to the papers "Lightwave Applications for Fiber Bragg Gratings", C. R. Giles, Journal of Lightwave Technology, Vol. 15, No. 8, August 1997, pages 1391 et seq., and "Fiber Gratings in Lasers and Amplifiers", by J. Archambault and S. G. Grubb, ibid. pages 1379 et seq.
It is well known that some of the characteristics of the aforesaid lasers are linked to the overall length of the cavity, which in a hybrid laser is given by the sum of the length of the cavity of the active element, of the distance between the anti-reflection coated facet and the tip of the tapered fiber end and lastly of the length of the fiber portion between the tip and the equivalent mirror plane of the grating. The equivalent mirror plane, as is well known, is the plane wherein a mirror would have to be positioned in order that a pulse sent by a source and reflected by the mirror returns to the source in the same time the pulse sent into the grating would take to return. In particular, the shorter the cavity of the laser, the greater the modulation band obtainable and the better the mode separation. It is evident that the attainment of good characteristics in terms of modulation band and mode separation is of particular interest for the use of lasers as sources for telecommunication systems.
The conventional low-reflecting Bragg gratings (with output reflectivity of the order of 70% currently used to form the external cavity of hybrid lasers have a profile of modulation of the refractive index that is symmetrical with respect to the central point in the grating, thus giving rise to an equivalent mirror plane positioned substantially at the center of the grating. On the other hand highly reflective gratings--with substantially 100% reflectivity--cannot be used, for the external cavity of the laser even if they would in themselves have an equivalent mirror plane offset towards one end because they would not allow sufficient power in the fiber.
The gratings used for these applications have a length of the order of a centimeter and thus the length of the external cavity constitutes nearly the entirety of the length of the whole cavity, since the active element has a cavity length of the order of 200 .mu.m. The use of conventional gratings may then give rise to a cavity length that is not sufficiently limited to obtain satisfactory characteristics for the laser. One could think of reducing the drawback by writing the grating in the end portion of the fiber, but this gives rise to additional problems when the fiber is fastened by means of resins onto the support of the module. It is evident that one of the fastening points must be in correspondence with the end portion, to guarantee the constant alignment between the active element and the fiber, and under such conditions the resin interacts with the grating. Experience has shown that the resin, upon curing, causes alternations in the structure of the grating, thus rendering the solution unfeasible.